The nutritive quality of proteins must be known before protein requirements can be estimated or the adequacy of diets evaluated. All methods currently in use have very serious deficiencies which invalidate the conclusions to greater or lesser degrees, although the measurements of biological value or net protein utilization are commonly thought to be satisfactory and to yield results comparable to amino acid score. Amino acid score is based upon the supposition that comparable degrees of deficiency of any essential amino acid will yield the same biological effect. This is not true. The overall objective of these studies is to develop adequate methods for the evaluation of nutritional quality of dietary proteins. Specifically, it includes extension of the previously proposed method for the evaluation of the efficiency of proteins for growth and an attempt to explain the results obtained from the amino acid composition of the proteins so far studied, investigations of the effects of each essential amino acid upon protein synthesis and degradation in adult rats, the definition of the maintenance requirements for each essential amino acid, and comparative studies on the efficiency of various proteins for growth and maintenance in rats and infants monkeys.